The present invention relates to calibration of signals in a wireless communication system. In particular, the present invention relates to calibrating receive path gain in a satellite communication system.
In many communication systems, one method of communicating involves time-division multiplexing (xe2x80x9cTDMxe2x80x9d). With TDM, data is ordered into regularly repeating time slots according to a data transfer standard, such as, for example, the asynchronous transfer mode (xe2x80x9cATMxe2x80x9d) standard. Time division multiple access (xe2x80x9cTDMAxe2x80x9d) systems divide the radio spectrum into time slots, and in each slot only one terminal of many is allowed to transmit. Each terminal is assigned a cyclically repeating time slot, and a channel may be thought of as a particular time slot that reoccurs every N-time slot frame in a particular frequency band. The number N depends on several factors specific to each communication system and may vary widely.
TDMA systems typically transmit data in a buffer-and-burst method, thus the transmission for any terminal is noncontinuous. Therefore, unlike frequency division multiple access (xe2x80x9cFDMAxe2x80x9d) systems (which can accommodate analog signals), digital data and digital modulation techniques are almost exclusively used with TDMA.
In a time division communication system, there are generally two types of slots, data slots and non-data slots. In this context, data slots refer to time slots in which terminals generally transmit xe2x80x9cusefulxe2x80x9d user information. The non-data slots in this context refer to time slots containing overhead information, for example, framing information, routing information, timing information, and requests for service. A frame may include a preamble, information message (a sequence of a predetermined number of time slots), and trail bits.
When signals transmitted in an uplink arrive at a satellite, present systems use a demodulator, including an analog to digital converter (xe2x80x9cADCxe2x80x9d), to process the signals. In the past, the ADCs used with such systems were unduly complicated, expensive, and power consuming, however, and had as many as nine bits of output or more. Past systems required complicated ADCs in part because a portion of the signal input to the ADC was variable gain in the receive path electronics of the satellite.
The receive path gain varies widely and is generally unknown, thereby requiring an increase in the ADC""s dynamic range (and number of output bits). Thus, ADCs with as many as nine bits of output were needed to cover the total possible dynamic range of input signal levels in the face of unpredictable receive path gain.
Unfortunately, the use of an ADC with a large dynamic range results in a more complex and power consuming ADC and also adds to the number of bits that must be carried into the digital processing system downstream of the ADC. Therefore, the use of a more complex ADC increases the cost of both the ADC and the downstream digital processing system.
In a multiple access system such as a TDMA system, the dynamic range of the uplink depends primarily upon the number of transmitting terminals and the gain variation in the RF receive path (which typically includes one or more amplifiers and additional receive path electronics). The only controllable factor of these two is the receive path gain variation. Unfortunately, past wireless communication systems did not provide a method of adequately controlling the gain variation in the receive path.
Some past systems use Automatic Gain Calibration (xe2x80x9cAGCxe2x80x9d) in attempting to alleviate some of the problems noted above. AGC does not, however, operate correctly in TDMA systems because of the dynamic loading of the input signal. In other words, the received signal power at the receiver (e.g., a satellite antenna) varies with the number of active terminals. Because the number of terminal signals simultaneously present at the input is not constant, the received signal power at the satellite antenna varies over time. As a result, the AGC circuit cannot differentiate between gain due to receive path electronics and power due to a large number of active terminals. Therefore, AGC techniques were unable to solve the problems noted above.
A need has long existed in the industry for a method of and apparatus for controlling gain variation in the receive path. Additionally, a need has long existed for a method of and apparatus for calibrating receive path gain, thereby permitting a less complex ADC to be used.
It is an object of the present invention to reduce the required dynamic range of an analog to digital converter in a wireless communication system demodulator.
It is another object of the present invention to provide an accurate gain measurement for the receive path of a satellite uplink.
It is an additional object of the present invention to provide a gain measurement for a satellite uplink that would allow for calibration and testing of any hardware component on the satellite.
It is a further object of the present invention to provide an improved analog to digital converter for use with demodulators in wireless communication systems.
It is yet another object of the present invention to enable a reduced complexity analog to digital converter to be used with demodulators in satellite communication systems.
It is a further object of the present invention to reduce the number of bits carried in the digital processing system downstream of an ADC.
It is a still further object of the present invention to calibrate receive path gain using an uplink signal without loss in bandwidth.
One or more of the preceding objects, or one or more other objects which will become plain upon consideration of the present specification, are satisfied, at least in part, by the invention described herein.
One aspect of the invention, which satisfies one or more of the above objects, is an apparatus for calibrating gain in satellite uplink receiver electronics. The apparatus includes an uplink receiver, a measurement processor, and an attenuator. The uplink receiver receives a satellite uplink and outputs an uplink signal to the measurement processor.
The measurement processor receives the uplink signal and includes circuitry to sample the uplink signal during a blanking interval (i.e., during a period of no transmission over the uplink, such as an empty slot or an empty time probe slot 306a in a TDMA uplink structure).
The measurement processor may connect to a snapshot buffer and include logic to sample the uplink signals periodically (e.g., approximately every thirty minutes) and may sample the uplink signals responsive to extreme changes in conditions). The measurement processor may also include circuitry that samples the uplink signals multiple times during a blanking interval to produce a multiple measurement sample (e.g., a sample average).
The measurement processor outputs a gain calibration (e.g., a sample average) which the attenuator uses to attenuate or calibrate the uplink signal to produce a calibrated signal. The attenuator may be, for example, a variable attenuator responsive to a gain calibration signal.
The apparatus may thereby use a lower complexity ADC coupled to the calibrated signal. The converter may have approximately 42 dB of dynamic range, for example, corresponding to 7 bits or less of output.
Another aspect of the invention is a method for calibrating gain in satellite uplink receiver electronics to reduce the dynamic range of the received uplink signals. The method includes the steps of receiving an uplink signal during a scheduled blanking interval, measuring gain associated with the blanking interval, and generating a calibrated signal by responsively controlling a variable attenuator in accordance with the measured gain. The method may sample the uplink signals periodically (e.g., every thirty minutes) and may sample the uplink signals responsive to extreme changes in conditions. The method may also include taking multiple samples during a blanking interval and averaging the multiple samples.